In the aforementioned copending application there have been described various circuits for providing a time delay between a direct current electric motor and an energizing source. In general, it has been found to be advantageous in the control of a direct current electric motor, i.e. the speed and torque thereof, to control the increase in voltage applied thereto by a time-delay means which maintains a minimum time interval between increases in the applied voltage.
Such circuits are employed because the application of an excessive high voltage, i.e. the application of a high voltage prematurely, at the terminals of the electric motor, causes a higher current to flow than is desirable and may result in burning out of the motor. The tolerable applied voltage thus is a function of the back e.m.f. resulting from rotation of the armature and hence the speed thereof. As a consequence the circuitry described above can avoid overenergization of the motor by delaying the application of the voltage increase, e.g. via the time-delay circuitry, until a sufficient back e.m.f. has developed or the armature has attained the requisite speed.
The time-delay networks become effective as soon as a potential is applied to the motor but problems are encountered when the motor is used for so-called motor braking.
In this operating mode kinetic energy of the vehicle is translated into rotation of the armature which is electromagnetically resisted by energization of the motor with a polarity that would, in the absence of this momentum, drive the motor in the opposite sense. In other words the armature rotates counter to its normal sense of rotation under the same polarity.
When the earlier time-delay systems were provided for a motor capable of motor braking, the time-delay networks permitted the applied voltage levels to become effective without limitation as long as the armature is inertially rotated in the opposite sense from that normally associated with the applied polarity. As soon as the armature came to standstill the time-delay networks, still conductive, applied high voltage directly to the motor so that a higher current flowed than was permissible, there having been no development of a sufficient back e.m.f. or opportunity for the armature to rotate in its normal sense.
As a consequence, in spite of the provision of time-delay switching to prevent application of prematurely high voltages, if such high voltages nevertheless are applied then there is a transition from direct-current or motor braking to normal drive operating unless, concurrently with the reduction of the armature speed to zero, there is a reduction in the applied potential. Frequently, the vehicle operator is incapable of creating this condition and motor burnout or other disruption of the system may occur.